Edema is defined as the abnormal accumulation of fluid in connective tissue. Edema typically results from a combination of passive venous congestion and salt and water retention, and may be systemic or localized to a particular region of the body. Dependent edema, in which fluid accumulates in the tissues of the limbic extremities, e.g. ankle, foot and the like, is a physical manifestation of a number of different human disease conditions. Dependent edema first appears in the feet and ankles of the ambulatory patient, and in the posterior surface of the calves and skin overlying the sacrum in the bedridden patient. Disease conditions characterized by the presence of dependent edema include local venous or lymphatic obstruction, cirrhosis, hypoalbumenia, and congestive heart failure.
In congestive heart failure, the presence of edema in the lower extremities is a valuable diagnostic marker for the presence of the disease. In addition to serving as a marker for the presence of congestive heart failure, the progression of the edemic state can be monitored over time and the progression of the edemic state related to the progression of the disease.
One way of detecting the presence of edema is to determine fluid volume change of the patient. A number of different technologies have been developed to identify the volume change, and include those based on the use of water or air-filled cuffs, mercury strain gauge, fiber optic strain gauge, and airborne ultrasound. Such technologies have principally been employed to measure venous blood flow and to sense the volume pulsations created by the heart.
Another method for detecting the presence of edema in a patient is the "pitting" method. In this method, a physician's thumb or finger is pressed into the patient's skin next to a bony surface (e.g., tibia, fibula, or sacrum). When the physician's finger is withdrawn, an indentation persists for a short time. The depth of the "pit" is estimated and generally recorded in millimeters, although subjective grading systems (e.g. ".sup.+++ ", etc.) have also been described. In general, the distribution of edema is also noted, as the amount of fluid is roughly proportional to the extent and the thickness of the pit.
Because dependent edema is a physical manifestation of a number of different disease conditions, the development of accurate methods for the detection of edema is of interest. Of particular interest is the development of methods which are sufficiently inexpensive and simple so as to be amenable to use in both conventional and out-patient health-care settings.